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Introduction
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The
Low Voltage Directive can usually be met by good design practice as
outlined in relevant harmonised standards. However, these standards
also describe tests which should be used to verify product compliance.
The five most widely used standards are: EN60335 covering household
and similar appliances; EN60065 which covers TVs, radios, audio equipment,
etc; EN60950 covering information technology equipment; EN61010 covering
the safety systems of electrical equipment for measurement and control;
and finally EN60204 covering the machinery directive. The paragraphs
below give a general description of some of the testing required,
but are based on EN60335. |
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Information
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Electric
shock
Access to live parts is the most obvious aspect of electrical safety.
This is judged using a series of standard probes and test fingers
(defined in IEC1032) which are designed to simulate what is most likely
to happen in practice.
Thermal hazards
Heating tests not only ensure that the user is protected from thermal
hazards, but also that internal components are not stressed beyond
their rating. Secondary problems such as degradation of insulation
and distortion of parts during normal operation should be detected
and prevented. There are also a series of tests for resistance to
heat, fire and tracking. This is a tree of sequential tests for insulating
material (plastics, printed circuit boards, coil formers, and so on)
with the number of branches being dependent on the application of
the material. A piece of decorative trim on the outside of an appliance
receives the least testing, but an insulator supporting connections
carrying more than 0.5A in an appliance intended for continuous unattended
use has to go through every test.
Rated currents
There are also tests to check that the input power to the appliance
matches the rating declared by the manufacturer. The purpose is mainly
to ensure that the appliance will not cause a problem for the building
wiring to which it will be connected.
Measurement of leakage current or insulation resistance is carried
out at normal working temperature and after a period (normally 48
hours) of exposure to high humidity. Further tests are carried out
on the insulation by subjecting it to electric strength tests. Here
a stress, normally between 1250 and 3750 volts, is applied to the
insulation after humidity treatment. This ensures that the humidity
has not created tracking paths and that the insulation will withstand
the occasional spike that may occur on the supply mains. No product
is required to withstand repeated electric strength tests - they should
only be performed if there is a specific reason to suspect that there
is an insulation problem that has not been detected by insulation
resistance measurement.
User misuse
Abnormal operation tests are an attempt to foresee what may go wrong
with an appliance and what problems user carelessness may cause. For
heating appliances, it is anticipated that the supply voltage may
deviate outside the ±6% range normally assumed. Motor operated
appliances are assumed to suffer from overloaded or stalled motors.
Covering of air inlet or outlet vents is a common test of misuse.
Mechanical aspects
Mechanical safety also takes normal and abnormal use into account.
Stability, and the enclosure of moving parts are naturally covered.
Abnormal testing includes checking that the appliance does not break
too easily if subjected to blows from the 0.5J impact hammer. More
severe tests are applied where relevant. Dropping hand held appliances
is an understandable requirement, but hitting the door of a microwave
oven with a 50mm steel ball may not be quite so obvious. Mechanical
aspects of electrical safety include requirements for screws and other
fastenings for electrical connections.
Warnings
The inclusion of warnings in the users instruction book is sometimes
an opportunity to cover a potential hazard that is difficult to alleviate
by mechanical means. It is usually mandatory to cover situations that
are inherent in the use of the product or which cannot be brought
to the user’s attention in any other way.
Components
LVD standards place a heavy emphasis on component approval and tests
carried out at component level and certified by the manufacturer will
usually reduce the testing required on your final product. Simple
devices such as capacitors or transistors can potentially have a large
number of different operating conditions, and testing each against
all the possible fault scenarios can be time consuming. Engineering
judgement should be used to determine which combinations of operating
mode and fault are most likely to create a hazard. |
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